This invention relates to the recovery of oil from subterranean oil reservoirs and more particularly to improved waterflooding operations involving the injection of an aqueous low tension slug in which the common ion effect is employed to suppress the divalent concentration thereof.
In the recovery of oil from oil-bearing reservoirs, it usually is possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the reservoir. Thus a variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean reservoirs. The most widely used supplemental recovery technique is waterflooding which involves the injection of water into an oil-bearing reservoir. As the water moves through the reservoir, it acts to displace oil therein to a production system composed of one or more wells through which the oil is recovered.
It has long been recognized that factors such as the interfacial tension between the injected water and the reservoir oil, the relative mobilities of the reservoir oil and injected water, and the wettability characteristics of the rock surfaces within the reservoir are factors which influence the amount of oil recovered by waterflooding. Thus it has been proposed to add surfactants to the injected water in order to lower the oil-water interfacial tension and/or to alter the wettability characteristics of the reservoir rock. Also, it has been proposed to add viscosifiers such as polymeric thickening agents to all or part of the injected water in order to increase the viscosity thereof, thus decreasing the mobility ratio between the injected water and oil and improving the sweep efficiency of the waterflood.
Processes which involve the injection of aqueous surfactant solutions in order to reduce the oil-water interfacial tension are commonly referred to as low tension waterflooding techniques. To date one of the more promising low tension waterflooding techniques involves the injection of aqueous solutions of petroleum sulfonates within a designated equivalent weight range and under controlled conditions of salinity. For example, in a paper by W. R. Foster entitled "A Low-Tension Waterflooding Process", JOURNAL OF PETROLEUM TECHNOLOGY, Vol. 25, Feb. 1973, pp. 205-210, there is disclosed a low tension waterflood process which involves the sequential injection of a protective slug to displace reservoir waters, a surfactant slug containing a petroleum sulfonate surfactant system, and a mobility control slug containing a suitable thickening agent. The surfactant slug contains sodium chloride in a concentration, typically about 1-2 weight percent, which will promote the desired low interfacial tension between the surfactant solution and the reservoir oil.
Another waterflooding procedure in which surfactants are formed in situ to reduce the oil-water interfacial tension involves alkaline waterflooding. In this procedure, an aqueous solution of an alkali metal or ammonium hydroxide or carbonate is injected in order to neutralize organic acids in the reservoir to produce the corresponding surfaceactive alkali metal or ammonium soaps. An improved alkaline waterflooding process is described in U.S. Pat. No. 3.927,716, issued Dec. 23, 1975 to Burdyn et al. In this process, an aqueous alkaline solution is employed in which the alkalinity and monovalent salt salinity are controlled in order to arrive at the desired low oil-water interfacial tension.
One problem encountered in low tension waterflooding techniques resides in the tendency of the surfactants to become adsorbed from solution onto the rock surfaces of the reservoir. To overcome this it has been proposed to employ inorganic sacrificial agents which decrease surfactant adsorption by satisfying adsorption sites within the reservoir. For example, as disclosed in the aforementioned Foster paper and also in U.S. Pat. No. 3,469,630 to Hurd et al., inorganic salt such as sodium tripolyphosphate and/or sodium carbonate may be employed to reduce surfactant adsorption. The sacrificial agents normally are injected into the reservoir in a pretreatment slug ahead of the surfactant slug and may also be included in the low tension slug itself.
The adverse effect of divalent alkaline earth metal ions in low tension waterflooding processes is widely recognized in the prior art. For example, in the processes disclosed in the aforementioned Foster paper and the Burdyn et al. patent, protective slugs may be employed to displace reservoir waters containing unacceptably high divalent metal ion concentrations ahead of the surfactant. U.S. Pat. No. 3,175,610 to Osoba recognizes that bivalent cations such as calcium and magnesium ions in the flooding solution have an adverse effect upon the ability of surface-active agents, whether formed in situ or added to the floodwater, to lower the interfacial tension. The Osoba patent teaches the use of mass action relationships to decrease divalent metal ion concentrations by initially flooding the formation with a solution that is higher in sodium chloride concentration than the formation water.
As noted previously, it is well known in the art to adjust the monovalent salt salinity of a low tension slug in order to arrive at the optimum low oil-water interfacial tension. While as a practical matter sodium chloride has usually been proposed for use in this regard, various other salts have also been suggested. For example, U.S. Pat. No. 3,373,808 to Patton discloses the use of ionizable sulfate compounds in combination with various surfactants including anionic surfactants such as petroleum sulfonates or nonionic surfactants such as polyoxyethylene-type surfactants. The ionizable sulfate compound may be employed in a slug injected ahead of the surfactant slug as well as in the surfactant slug. Patton specifically suggests the use of sodium sulfate and discloses experimental data showing that the lowering of interfacial tension achieved by the addition of sodium sulfate is greater than that obtained by the addition of sodium chloride in concentration ranges up to about 4.5 weight percent.
Yet another process in which salts other than sodium chloride may be employed in conjunction with surfactant waterflooding is disclosed in U.S. Pat. No. 3,369,602 to Fallgatter et al. This patent, while indicating that sodium chloride is generally preferred for economic reasons, suggests that salts such as sodium citrate, sodium sulfate, sodium phosphate, sodium silicate, sodium carbonate, sodium borate, as well as various other salts such as aluminum chloride, ammonium chloride, magnesium sulfate and potassium carbonate, may be employed. In the Fallgatter et al. process, the salts may be injected either before or after the surfactant slug in a particular sequence which is said to increase oil recovery.